By now, we can all name lots of things that are making Americans fat: McDonalds, video games, fad diets that don’t work… the list goes on and on. But here’s one I bet you haven’t heard before: using your brain.

How could using your brain possibly make you fat?
Trust me, as someone who spends most of their day thinking about science and planning and running experiments, this is the last thing that I want to hear, but unfortunately it’s true. Scientists at Laval University in Quebec demonstrated that test subjects ate significantly more from a buffet after they were asked to perform an intellectual task than when they were allowed to rest for the same period of time.

How did they know that the people weren’t just feeling more hungry for some other reason?
It’s hard to control for all the factors that go into how much a person eats, but they tried to eliminate as many of these variables as possible. First, they picked all women, in order to rule out any differences between the genders (don’t worry guys, they’re planning a bigger experiment that will include you too), and all subjects were of a similar weight and waist measurement. They also surveyed people and ruled out those with abnormal stress levels or eating habits to prevent those factors from affecting their results.

They started these experiments first thing in the morning and fed everyone the same amounts of the same breakfast, followed by a period of mental activity or rest, and then let them eat from a buffet. They repeated this on three different days for each person, having them rest once and engage in two different mental activities: reading a passage and summarizing it and taking computerized tests. They also shuffled the order of these activities for each participant. Using these methods, they could compare how much the same person ate under each of the three conditions, having had the same amount of food for breakfast beforehand.

So what exactly did they find?
They found that after a period of rest, people consumed an average of 860 calories, after reading and writing they consumed an average of 1063 calories, and after taking computerized tests they consumed an average of 1113 calories. That means that intellectual work made the same person consume, on average, about 200-250 more calories. This is problematic, because the actual mental work itself burnt only 3 calories more than sitting around doing nothing.

Then why would these people eat more calories?
When you eat, your body takes in calories in three forms: protein, fat and carbohydrates. Different diets take different approaches to limiting these three things to help you lose weight, but in moderation, all three of them are important for different processes in your body. Luckily the body is able to use energy from each of these sources to produce the others, in an attempt to keep everything it needs in balance.

When you are engaged in a physical task your body is able to burn calories from a wide range of sources, but the brain relies on carbohydrates, specifically a small sugar called glucose, as an energy source. The human body isn’t able to store large pools of glucose and tends to burn through them rapidly when it needs a quick source of energy. As I mentioned, the body can replenish the store of glucose by producing it with energy from other sources of calories, but this process may trigger a desire to take in more calories, even if you don’t really need them.

So when you think a lot, your brain burns the stored glucose and triggers a desire to eat more?
That’s what the authors of this study believe. To try to prove this, they took blood samples from each person throughout the period of mental work or rest and measured the glucose levels to see how big the available pools were. They found that people doing mental work showed fluctuations in glucose levels, while people resting had fairly stable amounts. However, this study was pretty small, so they need to expand it to include more people in order to know for sure what’s going on.

THE BOTTOM LINE
Scientists found that when given mental tasks, people ate 200-250 more calories from a buffet than they did when resting their brains for the same duration of time. Does this mean that you should stop doing activities involving thinking? No. If you’re eating a healthy diet and getting some exercise, you should be just fine.

LINKSStudy abstract – A short description of the study from the Journal of Psychosomatic MedicineNews article – A summary of the study from EurekAlertBrain energy usage – A little snippet from Wikipedia, showing a scan of a human brain and its energy consumptionMental agility and nutrition – Another Wikipedia snippet with some references to studies linking proper nutrition to learning